The present invention relates to a fluorescent conversion filter used in combination with a light emitting element in a color display devices. The present invention is useful in commercial and industrial uses such as a light-emitting-type multicolor displays, a light-emitting-type full-color display, a color display panel, a monochromatic signal-light display panel and a back light. Specifically, the present invention relates to a fluorescent conversion filter that converts light in the region between near ultraviolet and green to light in the red region and facilitates its very fine patterning. More specifically, the present invention relates to a color display device that includes a fluorescent conversion filter as described above.
Research and development of various light emitting elements have been explored vigorously to meet the increasing demands for flat panel displays. Flat panel displays have become popular replacements for conventional cathode-ray-tube (CRT) displays because they save space and reduce power consumption. The electro-luminescent element (hereinafter simply referred to as a "light emitting element") is an all-solid-state self-light-emitting element that meets the above described demands. The electroluminescent element has attracted much attention due to its very high resolution and very high visibility, which other display devices do not exhibit.
To provide a flat panel display with a multicolor display function (or a full-color display function), light emitters of three primary colors (i.e. red-light emitters, blue-light emitters and green-light emitters) are separately arranged in a matrix. The light emitters in the matrix are separately controlled to emit light in each respective color (cf. Japanese Unexamined Laid Open Patent Applications No. S57-157487, No. S58-147989 and No. H03-214593). It is technically difficult and expensive to use an organic light emitting element for a color display. This is primarily due to the fact that three kinds of light emitting materials (one for each of the three primary colors) must be arranged very finely in a matrix. Each of the three light emitting elements has a different life in which the color remains pure. Since the lives of the three different light emitting materials are not identical, chromaticity deviations result over elapsed time.
The three primary colors are obtained by transmitting white light from a back light through color filters (cf. Japanese Unexamined Laid Open Patent Applications No. HOI -315988, No. H02-273496 and No. H03-194895). In order to achieve the three primary colors at high luminance it is necessary to provide a white light with a high luminance. However, it has not yet been possible to obtain an organic light emitting element which emits white light at a high luminance for a long-life.
Japanese Unexamined Laid Open Patent Application No. H03-152897 discloses a planar and separate arrangement of fluorescent materials which absorbs light from a light emitter and emits polychromatic fluorescent light. The planar and separate arrangement of fluorescent materials for emitting polychromatic fluorescent light is applicable to both CRT and plasma type displays. Japanese Unexamined Laid Open Patent Applications No. H03- 152897 and H05-258860 disclose a color conversion method that uses fluorescent materials. Flourescent materials can absorb the light emitted from an organic light emitting element and emit fluorescent light in the visible wavelength region. Since the organic light emitting element can emit colored light, the organic light emitting element which emits light at a higher luminance may be used as a light source. Japanese Unexamined Laid Open Patent Applications No. H03-152897, No. H08-286033 and H09-208944 disclose a color conversion method that uses an organic blue-light emitting element as a light source and converts the blue light to green light and red light by respective fluorescent pigments.
By finely patterning fluorescent conversion films, each including one fluorescent pigment, a full-color light-emitting type display may be constructed. The finely patterned conversion films provide a full-color display even when a weak energy ray (such as a near ultraviolet ray and a visible ray from a light emitter) is used. The fluorescent conversion filter is patterned in a similar manner as an inorganic fluorescent converter. First, a fluorescent pigment is dispersed in a liquid photo-resist (photo-reactive polymer) to form a dispersion liquid. Second, the dispersion liquid is spin coated to form a film. The formed film is patterned by photo-lithography techniques (cf Japanese Unexamined Laid Open Patent Applications No. H05-198921 and No. H05-258860). The fluorescent conversion filter is also patterned by dispersing a fluorescent pigment into a basic binder and etching the basic binder film including the fluorescent pigment with acidic aqueous solution (cf. Japanese Unexamined Laid Open Patent Application No. H09-208944).
An organic flourescent pigment is dispersed in a liquid photo-resist. The liquid phot-resist includes a photo-polymerization agent or a thermosetting agent (polymerization initiator), a reactive multi-functional monomers and a reactive multi-functional oligomers. During the photo-lithography process for patterning, bleaching of the organic pigment and extinction are often caused by the radicals and ions produced from the photo-polymerization agent or thermosetting agent (polymerization initiator), reactive multi-functional monomers and reactive multi- functional oligomers.
As described above, photo-resist is conventionally coated on a fluorescent conversion films which includes a basic binder and the photo-resist is patterned before etching with an acidic aqueous solution. This conventional method results in defects in the photo-resist which adversely affect the fluorescent conversion film. This problem has not been yet been solved.
High purity red light cannot be not obtained by a conventional flourescent conversion film which has an absorbance of less than 1. When the absorbance of the fluorescent conversion film is less than I, more than 10% of the light from the blue- or green-light emitting element is not absorbed.
Generally, adding too much fluorescent pigment to a red color filter causes self-absorption and concentration extinction. Although it is possible to achieve a highly pure red light by adding flourescent pigment, low conversion efficiency results when the pigment concentration in the red color filter is high enough to have an absorbance of greater than 1 in the wavelength region between 450 nm and 520 nm. Alternatively, the fluorescent pigment concentration can be reduced and the thickness of the color conversion filter can be increased so that an absorbance of greater than 1 may be obtained in the wavelength region between 450 nm and 520 nm. However, the thickened color conversion filter causes light leakage to the adjacent pixels, resulting in a narrowed angle of visibility.
In manufacturing color conversion type displays, the distance between the fluorescent conversion filter and the organic electro-luminescent (EL) element should be carefully adjusted. Light leaks to adjacent pixels when the distance between the fluorescent conversion filter and the organic EL element is long. Thus, a long distance between the flourescent conversion filter and the organic EL element results in a narrowed angle of visibility. Conversely, a shorter distance between the fluorescent conversion filter and the organic EL element results in a wider angle of visibility. In order to produce a wide angle of visibility, it is desirable to form an organic EL layer which is in close contact with an upper surface of a color conversion filter.
Japanese Unexamined Laid Open Patent Application No. H08-279394 discloses a conventional method for forming a transparent and electrically insulating inorganic layer directly onto fluorescent layers. Because it is difficult to flatten the color steps on the fluorescent conversion filter, undesirable concave and convex portions are found on the as-formed electrically insulating inorganic layer. As a result of this, the organic light emitting element is inevitably formed on the concave and convex portions, causing unfavorable influences which prevent very fine display performances.
The above described problem can be reduced by coating a protection layer on the fluorescent conversion filter. The protection layer flattens the surface on which the electrically insulating inorganic oxide layer is later formed. However, since the thickness of the protection layer changes with the thickness of the fluorescent conversion films, the protection layer for flattening a thick fluorescent conversion film is inevitably thick. Because of the increased thickness of the protection film, the angle of visibility of the light emitting element is narrowed.
According to manufacturing methods, the above described protection film is hardened by repeatedly heating the fluorescent conversion films. The flourescent conversion film and the protection film have different linear heat expansion coefficients. When the glass transition temperature of the fluorescent conversion film is low, the heating steps result in pattern distortions due to the difference in the linear expansion coefficients of the films. To address this problem, the protection layer materials must be carefully selected to have a low hardening temperature, harden by ultraviolet light or harden by visible light. However, even when a fluorescent conversion filter is manufactured using materials selected according to the above guideline, pattern distortions may still result. Since the glass transition temperature of the flourescent conversion film is low, the fluorescent conversion filter will have pattern distortions caused by high temperature testing.